rduPilotMega is a full-featured IMU-based autopilot that can be used for either airplanes or rotary wing aircraft, including quadcopters and other multicopters and traditional helicopters. The autopilot hardware is the same across all aircraft, and you can load whichever firmware is appropriate for the vehicle you've got via the free Mission Planner. (You may choose to add other sensors to improve performance for specific airframes, such as an airspeed sensor for airplanes or a magnetometer for mulitcopters).